Driving a laptop with a car's battery.

[alphacat]

Hey,

I have a laptop which its battey is charged from a 5V charger.
I'm not sure though how much current it draws from the charger.
(I dont have the laptop here so i cant tell you its type and firm).

Could you tell me please if its possible to drive a laptop from the car's battery?
Meaning if the car's battery can supply the current that the laptop requires?

The car's battery is 12V-DC, right?
I though of using this 5V regulator, NCP1117ST50T3G:https://www.onsemi.com/pub_link/Collateral/NCP1117-D.PDF

It is said in the datasheet, that for Vin = 6.5V to 12V and Iout = 0A to 0.8A, the output voltage is 4.9V to 5.1V.
Do you think it could do the job without getting too hot?
I want the car's battery to drive it for 2-3 hours.

Thanks for any help.

 

[blueroomelectronics]

Why not just buy a car charger made for your laptop like those made by Targus. Many laptops draw several amps, I doubt 0.8A is nearly enough.

 

[Nigel Goodwin]

Why not just buy a car charger made for your laptop like those made by Targus. Many laptops draw several amps, I doubt 0.8A is nearly enough.

No where near enough - as you suggested Bill, he should just buy one, commercial ones are reasonable prices, and switch-mode to keep cool.

 

[alphacat]

I wanted to build such PSU by myself (and due to the other thread, its not becaues i wanna spare a few bucks, its a project i took on myself).

Is there anyway to use a few regulators or mosfets/transistors, maybe connecting them in parallel, in order for them to provide the laptop with the required current?

How much current a laptop consumes in general?

 

[Nigel Goodwin]

I wanted to build such PSU by myself (and due to the other thread, its not becaues i wanna spare a few bucks, its a project i took on myself).

Is there anyway to use a few regulators or mosfets/transistors, maybe connecting them in parallel, in order for them to provide the laptop with the required current?

How much current a laptop consumes in general?

Something like 6-8A - it's a LOT.

You need a high-current switchmode regulator, EPE magazine did a design a year or two ago. To do it with linear regulators is going to be really HOT!!.

 

[alphacat]

thanks

I though of a simple idea but i'm not sure if it would work.
Is it possible to connect a resistor in series with the car's battery, like a source impedance, and connect the laptop to the resistor?

This is how i calculated the resistor's value.
Assuming that my laptop draws 7A.
The resistor's voltage drop should be 7V (=12V - 5V).
Therefore, the resistor's value should be 7V / 7A = 1Ω.
And the resistor's power dissipation should be 49W.

(Of course that I can connect several larger resistors in parallel, so their equivalent resistance equals to 1Ω, and that way they will disipate less power and will get less hot).

What do you think?

 

[blueroomelectronics]

Nope your laptop wants a regulated DC supply. It's power draw will vary.

 

[alphacat]

I see thanks.
I guess I'll have to put some thought about it and read about high current switch mode regulator designs.

By the way, how is a laptop charger built?
I assume that it converts tha AC mains to a DC voltage using a transformer and half/full wave rectifiers + output capacitor,
but does it convert the AC mains voltage directly to 5VDC or does it convert it to some higher value and then uses a regulator in the midway?

 

[blueroomelectronics]

It's more a power supply than a charger, very similar to any computers SMPS. Your laptop will have an internal charger that'll shutoff the supply voltage when the battery is full.
https://en.wikipedia.org/wiki/Switched-mode_power_supply

 

[smanches]

All modern computer supplies are off-line buck converters in some flavor.

 

[alphacat]

Thanks Bill and Smanches.

I read a lot about SMPS.
I understand the structure of half/full wave rectifier with an output capacitor, and that afterwards there's usually a buck converter (discontinuous/continuous mode - depends on the required current).

My idea was that if this laptop charger uses a regulator/buck converter after the transfoermer & rectifiers, then I could disassemble the charger and drive the buck converter with the car's battery.
You think its possible to pull it off?

 

[blueroomelectronics]

You mean convert a household 110VAC SMPS to a 13.8VDC car version? Not likely.

 

[Nigel Goodwin]

Thanks Bill and Smanches.

I read a lot about SMPS.
I understand the structure of half/full wave rectifier with an output capacitor, and that afterwards there's usually a buck converter (discontinuous/continuous mode - depends on the required current).

My idea was that if this laptop charger uses a regulator/buck converter after the transfoermer & rectifiers, then I could disassemble the charger and drive the buck converter with the car's battery.
You think its possible to pull it off?

They don't use a mains transformer - that wouldn't be a SMPSU.

They rectifiy the mains, to give 320V DC (on 240V mains), this is then chopped using a switching transistor and applied to a small high frequency transformer. The output of that is rectified and smoothed, and used to feed an error voltage back to the switching transistor, which alters it's mark/space ratio to correct the voltage error.

 

[alphacat]

I see, thanks guys.

I was thinking of disassembling some charger that i could use its inside to convert the 12Vdc car's battery to 5VDC that the laptop requires.
You got any idea?

 

[blueroomelectronics]

I don't know of any laptop that's 5V, most are 16V to 19V.

Find out the model number and post it here.

 

[alphacat]

Alirght, thanks for all your help guys.